Navigation devices in which a portion of a road map is reproduced on a screen as a function of the position of a vehicle and its direction of motion are known from the existing art. A number of types of depiction are usual, divided into various categories in terms of their underlying data structures and the calculation operations carried out by the system. The simplest model is a two-dimensional depiction of a vector map containing the navigation data (surfaces, lines, points, labels). On the other hand, a perspective depiction of the map data is often preferred in order to make available to the person operating the vehicle visual information about his or surroundings that is as easy as possible to interpret. The depiction can optionally contain elevation information about the terrain (which is not necessarily flat), or can be merely a three-dimensional depiction of a “flat” map. The latter model is referred to as a pseudo-3D depiction. In both cases, the data of a vector map on which the map is based must be converted, under the influence of a coordinate transformation, into the image data containing a perspective representation. A linear imaging procedure is usually used, in which only the end points of lines present in the map are projected and are reconnected with a line. The polygons that are to be filled are then assembled from these lines. A standard CPU (central processing unit) or a GPU (graphical processing unit) carries out the necessary 3D vector operations and polygon shading operations in hardware.
The perspective impression is achieved by selecting a virtual viewing position above the actual vehicle position. The higher the viewing position, the more information is made available to the user—information which in some cases he or she would not obtain from his or her position in the vehicle. For better orientation, the position of the vehicle is often also reproduced on the map.
Any projection method that generates a three-dimensional effect can in principle be used to calculate the image data. Particularly well known in this context is central projection, in which all the points to be imaged in the map are connected via connecting lines to the projection center, which coincides with the viewing position, via connecting lines whose intersection points with a projection plane yields the projected image. Because of the geometry of this kind of projection or a similar one, a horizon and an adjacent region that corresponds to the sky appear in the visual depiction. Static, decorative textures, in particular cloud images, are often provided in the depiction of the sky. As the vehicle moves and the visual depiction is correspondingly updated, such textures appear unrealistic because of their static nature, since they do not follow the motion of the vehicle.